I would tell you to go with any of the frameworks. I personally feel good with codeigniter because I feel more comfortable with its level of documentations. There are several reasons why use frameworks. I would like list to few of them.

• MVC (model view controller) patterns. This actually keeps your model(data), view(templates or theme) and controller ( handle page requests and bind everything together) separate.
• Libraries and utilities. There are many things readily available to you. You should just use them. The documentations will probably help you a lot. So you don’t have to reinvent the wheels.
• Security. There are many things available to you in php so that you can just use them to control some certain attacks. But still there are chances that you may miss something. In frameworks many things are done for you automatically. For an instance consider SQL injections.
• caching, benchmarks, profiling. These things may be readily available for you with frameworks. That’s basically a performance related one.
• For a team work it great suits.

These are the things which I found good about frameworks. frameworks helps us write cleaner code. It is always a good thing to work on frameworks. Using frameworks will provide maintainability. I would recommend you codeigniter. But cakePHP is also fine. Have a look at this link. There is a good comparison between those two frameworks. Hope this helps.